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7:14 AM
@JohnRennie morning sir alesha here
do u have time to discuss some question which I have related to this
 
@JackRod Hi Alesha :-)
Yes, we can talk about this if you want.
 
a genuine critic of its explaination is
Close the switch, and at the exact same moment, cut the wire at the u-bend on the switch side.  The light won't light as the circuit was never actually complete.  Since any effect on the wire at a distance of 1/2 light seconds away won't be seen for 1/2 second,
there is no way for the light to light within a few nanoseconds of closing the switch. There is no magic teleportation of information from 1/2 light second away. (BTW - the time to lighting the bulb is about 1.3 seconds, considering propagation delays in copper wire.)
 
Let me suggest another way of looking at the circuit that I hope will make it a bit clearer.
The circuit consists of a very long straight wire with the battery at the centre, and a second very long straight wire 1 metre away with the bulb in the middle.
The way to look at this is like the primary and secondary of a transformer.
 
ok
 
A transformer works because the primary and secondary coils are linked by the EM field between them. You could build a transformer where the primary and secondary coils were a metre apart. In fact this is how wireless chargers work, although the separation there is usually only a few cm rather than a metre.
OK so far?
 
7:23 AM
yes
 
When the switch is closed current starts flowing from the battery in the wire near the battery, and this creates an EM field. That field reaches the other wire a time 1/c later, and it starts inducing a current in that second wire. This current then starts to light the bulb.
Note I say "starts" because the bulb doesn't immediately go to full brightness. The current through the bulb will change with time ina complicated way that I don't know how to calculate.
Now, you talk about cutting the wires at the edges of the circuit a light year apart, and you could do this. And the bulb would indeed still light briefly.
That's because the long wires now act like capacitors.
When the switch is closed current still flows because the wire is acting like a capacitor and current flows as charge flows onto the capacitor.
@JackRod Does this make sense?
 
yes but I have some small doubt
 
What's your question?
 
If the energy is transferred in the space extremely close to the conductor, and he said that electric field needs to extend through the circuit (at 6:15) , does it mean that he's wrong saying that the light bulb will turn on almost instantaneously (at 11:45)?
 
The trick here is the two long wires have been carefully laid out to be parallel and only 1m apart, so the field only has to cross the 1m gap between the two parallel wires. That's why the time taken is 1m/c.
 
7:33 AM
@JohnRennie ok
 
It's kind of a trick question. Signals do normally travel at about ¹⁄₂c in wires, but in this case the wires have been laid out in a slightly strange way.
 
I got it can u share some rough idea baout voltage
 
What about voltage?
 
what voltage would be between the source and bulb voltage in nanoseconds?
 
I don't know how to calculate the way the EMF across the bulb changes with time. Sorry :-(
 

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